When a concrete slab is poured larger than a particular size, cracks will form after setting of the concrete as the concrete cures due to unavoidable shrinkage in the concrete. For structural reasons, it is preferable for cracks to form at predetermined locations in the concrete slab. For thinner concrete formations, such as footpaths and the like, this is normally achieved by shaping a groove into the surface of the concrete with an edging tool. For thicker slabs of concrete, such an approach hardly ever works with the result cracks do not form where required and normally form where that are not required. Accordingly, a slot is normally cut into the concrete slab using a concrete cutting tool. Concrete cutting can only be carried out within a limited time frame after pouring and setting of the concrete, and is also costly, noisy, messy and not always effective. Cutting also creates a slurry which can cause environmental problems.
Once a crack has been formed in the concrete slab, even if it is propagated at a desired location, the concrete slab is no longer sealed against water transfer from one side to the other of the slab. Sealants are not normally set into in the concrete until the slab is twenty-eight days old because most of the shrinkage that the concrete undergoes occurs predominantly within such a time frame. Shrinkage normally continues significantly for twelve months or longer.
An attempt to address the issue of crack propagation has been made in the form of a crack inducer introduced into the base of a concrete slab at the time of casting. However, the cracks propagated by such a device diverge from the location of the crack inducer, and produce unsightly jagged cracks in the surface of the slab. It is almost always quite critical that reinforcing bars or reinforcing mesh be located more than a certain distance from the surface of the concrete not only to prevent spalling but also to provide maximum strength. Although bar chairs typically provide accurate spacing of reinforcing bars from the lower surface of a concrete slab, it is sometimes difficult to ensure that reinforcing bars are accurately spaced from the upper surface.
Attempts have also been made to mount crack inducers to reinforcing mesh. However, it has been found that such arrangements have undesirable effects when pouring the concrete, particularly when personnel walk through the liquid concrete mix, standing on the reinforcing mesh to which the crack inducers are mounted.
In this specification, terms reinforcing bar and the like may be taken to include reinforcing mesh, and the term reinforcing mesh is to be taken to include reinforcing bars and similar terms where the context so indicates. However, the term “separate reinforcing bar” refers to reinforcing bar which is separate from any reinforcing mesh.
The present invention aims to provide crack inducer apparatus which alleviates one or more of the problems associated with cracking of concrete slabs. Other aims and advantages of the invention may become apparent from the following description.